1) Field of the Invention
The present application relates to an optical device, and particularly to an optical device suitable for use in technical fields of optical communication, optical signal processing and so forth.
2) Description of the Related Art
It is known that, if an electric field is applied to dielectric crystal or ferroelectric crystal, then the refraction index thereof varies by an electro-optical effect. Conventionally, various optical devices such as an optical modulator, an optical deflection device, an optical switch and so forth have been developed making use of the effect just described. Here, are fraction index variation Δn by the electro-optical effect is represented by an expression (1) given below.
                              Δ          ⁢                                          ⁢          n                =                              -                          1              2                                ⁢                      n            3                    ⁢          rE                                    (        1        )            where n is the refraction index of the ferroelectric substance, E the applied voltage, and r the electro-optical coefficient. Where an optical modulator is taken as an example, LiNbO3 is used frequently as a material. The electro-optical coefficient of LiNbO3 is 30 pm/V, which is not a very high value in comparison with those of other electro-optical materials. SBN (Sr0.75Ba0.25Nb2O6), PLZT (Pb1-xLax(ZryTi1-y)1-x/4O3) and so forth are known as a ferroelectric material having a relatively high electro-optical coefficient.
A relationship between representative dielectric materials and electro-optical coefficients is illustrated in FIG. 8. It is to be noted that, in FIG. 8, BNN is Ba2NaNbO5, and BSTN is Ba1-xSrxTiyNb2-yO6. If an optical waveguide is formed from a material having a higher electro-optical coefficient, then a low-voltage optical device can be formed and reduction of power consumption as an optical device can be expected.
Here, where an optical device is formed from a material having such a high electro-optical effect as described above such as SBN, formation of an optical waveguide is a significant subject. In particular, while it is known that, where LiNbO3 is used, a low-loss optical waveguide can be formed by thermally diffusing a metal such as Ti, where a material such as SBN, PLZT or the like is used, it is hard to diffuse a metal. Therefore, in the method just described, the propagation loss increases and it is not easy to form a low-loss optical waveguide.
On the other hand, as an optical waveguide forming method, a stress-applied type waveguide is disclosed in O. Eknoyan. et al. “Strain induced optical waveguides in lithium niobate, lithium tantalite, and barium titanate”, Appl. Phys. Lett., vol 60, No. 4, 27 Jan. 1992, pp 407-409 (hereinafter referred to as Non-Patent Document 1). The waveguide disclosed in Non-Patent Document 1 is formed in the following manner. First, a stress film whose thermal expansion coefficient is different from that of a ferroelectric substrate which has a photoelastic effect is deposited on the substrate at a high temperature. At this time, stress is generated uniformly in the substrate by the difference between the thermal expansion coefficients of the substrate and the stress film. Then, when the stress film is patterned by etching, stress is generated at edges of the stress film thereby to apply distortion to the substrate. The refraction index is varied by the photoelastic effect of the ferroelectric substance thereby to form an optical waveguide.
In particular, Non-Patent Document 1 mentioned above discloses such an optical device 100 as shown in FIG. 9, wherein a stress layer 102 patterned as a film is formed on a substrate 101. In the optical device 100, distortion is generated in a region 103 between different portions of the stress layer 102 of the substrate 101 by stress at generated at edges of the stress layer 102. Then, since the refraction index of the region 103 in which the distortion is generated varies by the photoelastic effect, the region 103 can be formed as an optical waveguide (stress optical waveguide) 103A by the photoelastic effect.
Further, as a known technique relating to the present application, techniques disclosed in Patent Documents 1 to 3 are available. In Japanese Patent Laid-Open No. Hei 2-5028 thereinafter referred to as Patent Document 1), an Au pattern for stress application is disclosed. In Japanese Patent Laid-Open No. 2003-248130 (hereinafter referred to as Patent Document 2), a technique wherein an intermediate layer for moderating distortion by a thermal expansion coefficient difference between a core layer and a cladding layer, which form an optical waveguide layer, and a substrate is disclosed. Further, in Japanese Patent Laid-Open No. Hei 6-53312 (hereinafter referred to as Patent Document 3), a technique wherein a stress applying film is formed on both faces of a silicon substrate in order to cancel tension and contraction of a device separation pattern.